Total joint arthroplasty for end-stage joint diseases most commonly involves a metal/polymer articular pair. Polyethylene, particularly ultrahigh molecular weight polyethylene (UHMWPE), has been and remains the material of choice for the load-bearing, articulating surface for this articular pair for more than four decades (Kurtz, et al., Biomaterials, 1999. 20(18): p. 1659-1688). Despite high long-term success rates for such reconstructions, wear and fatigue damage of polyethylene limit the longevity of total joints. In total knees, implant failure is caused primarily by fatigue damage to the polyethylene components (Collier, et al., J. Arthroplasty, 1996. 11(4): p. 377-389). One solution to prevent osteolysis in total hips is cross-linking, which markedly reduces polyethylene wear (Muratoglu, et al., J Arthroplasty, 2001. 16(2): p. 149-160; Muratoglu, et al., Biomaterials, 1999. 20(16): p. 1463-1470; McKellop, et al., J Orthop Res, 1999. 17(2): p. 157-167).
Increased crosslink density in polymeric material is desired in bearing surface applications for joint arthroplasty because it significantly increases wear resistance. Oxidation-resistant cross-linked polymeric material, such as UHMWPE, is desired in medical devices because it significantly increases the wear resistance of the devices. A method of cross-linking is by exposing the UHMWPE to ionizing radiation. However, cross-linking also reduces the fatigue strength of polyethylene, therefore limiting the use of highly cross-linked polyethylenes in total knees where the components are subjected to cyclic loading accompanied by high stresses. Ionizing radiation, in addition to cross-linking, also will generate residual free radicals, which are the precursors of oxidation-induced embrittlement. This is known to adversely affect in vivo device performance. Therefore, it is desirable to reduce the concentration of residual free radicals, preferably to undetectable levels, following irradiation to avoid long-term oxidation.
One way of substantially reducing the concentration of residual free radicals in irradiated UHMWPE is to heat the irradiated UHMWPE to above its melting temperature (for example, about 137° C.-140° C.). Melting frees or eliminates the crystalline structure, where the residual free radicals are believed to be trapped. This increase in the free radical mobility facilitates the recombination reactions, through which the residual free radical concentration can be markedly reduced. This technique, while effective at recombining the residual free radicals, has been shown to decrease the final crystallinity of the material. This loss of crystallinity will reduce the modulus of the UHMWPE. Yet for high stress applications, such as unicompartmental knee designs, thin polyethylene tibial knee inserts, low conformity articulations, etc., high modulus is desired to minimize creep.
Cross-linking by irradiation decreases the fatigue strength of UHMWPE. In addition, post-irradiation melting further decreases the fatigue strength of the UHMWPE. Radiation and melting also decrease the yield strength, ultimate tensile strength, toughness and elongation at break of UHMWPE.
Melting in combination with irradiation creates cross-links and facilitates recombination of the residual free radicals trapped mostly in the crystalline regions, which otherwise would cause oxidative embrittlement upon reactions with oxygen. Both cross-linking and melting, however, decrease the crystallinity of UHMWPE. Cross-linking and decrease in the crystallinity is thought to be the reason for decrease in fatigue strength, yield strength, ultimate tensile strength, toughness and elongation at break. Some or all of these changes in properties limit the use of low wear highly cross-linked UHMWPE to low stress applications. Therefore, a cross-linked UHMWPE with higher crystallinity is desirable for low wear and high fatigue resistance for high stress application that require low wear.
It is, therefore, desirable to reduce the irradiation-created residual free radical concentration in cross-linked UHMWPE without reducing crystallinity, so as to achieve high fatigue resistance for high stress application that require low wear. Alternative methods to melting can be used to prevent the long-term oxidation of irradiated UHMWPE to preserve higher levels of crystallinity and fatigue strength.
The effect of crystallinity on the fatigue strength of conventional UHMWPE is known. Investigators increased the crystallinity of unirradiated UHMWPE by high-pressure crystallization, which increased the fatigue crack propagation resistance of unirradiated UHMWPE by about 25% (Baker et al., Polymer, 2000. 41(2): p. 795-808). Others found that under high pressures (2,000-7,000 bars) and high temperatures (>200° C.), polyethylene grows extended chain crystals and achieves a higher crystallinity level (Wunderlich et al., Journal of Polymer Science Part A-2: Polymer Physics, 1969. 7(12): p. 2043-2050). However, high-pressure crystallization of highly cross-linked UHMWPE has not been previously attempted or discussed. Also, the crystallization behavior of highly cross-linked polyethylene at high pressures has not been determined.
Polyethylene undergoes a phase transformation at elevated temperatures and pressures from the orthorhombic to the hexagonal crystalline phase. The hexagonal phase can grow extended chain crystals and result in higher crystallinity in polyethylene. This is believed to be a consequence of less hindered crystallization kinetics in the hexagonal phase compared with the orthorhombic phase. One could further reduce the hindrance on the crystallization kinetics by introducing a plasticizing or a nucleating agent into the polyethylene prior to high-pressure crystallization. The polyethylene can be doped with a plasticizing agent, for example, α-tocopherol or vitamin E, prior to high-pressure crystallization. The doping can be achieved either by blending the polyethylene resin powder with the plasticizing agent and consolidating the blend or by diffusing the plasticizing agent into the consolidated polyethylene. Various processes of doping can be employed as described in U.S. application Ser. No. 10/757,551, filed Jan. 15, 2004, and PCT/US/04/00857, filed Jan. 15, 2004, the entirety of which are hereby incorporated by reference.